- Slides: 14
LIQUID CRYSTAL DISPLAYS(LCD)
INTRODUCTION §The liquid crystal display has the distinct advantage of having a low power consumption than the LED. §It is typically of the order of microwatts for the display in comparison to the some order of milliwatts for LEDs. § Low power consumption requirement has made it compatible with MOS integrated logic circuit. §Its other advantages are its low cost, and good contrast. §The main drawbacks of LCDs are additional requirement of light source, a limited temperature range of operation (between 0 and 60° C), low reliability, short operating life, poor visibility in low ambient lighting, slow speed and the need for an ac drive.
BASIC STRUCTURE OF AN LCD §A liquid crystal cell consists of a thin layer (about 10 µm) of a liquid crystal sand wiched between two glass sheets with transparent elec trodes deposited on their inside faces. §With both glass sheets transparent, the cell is known as transmittive type cell. §When one glass is transparent and the other has a reflective coating, the cell is called reflective type. The LCD does not produce any illumination of its own. § It, in fact, depends entirely on illumination falling on it from an external source for its visual effect
TYPES OF LCD §Dynamic scattering display: When dynamic scattering display is energized, the molecules of energized area of the display become turbulent and scatter light in all directions. § Consequently, the activated areas take on a frosted glass appearance resulting in a silver display. Of course, the unenergized areas remain translucent. Field effect LCD: Field effect LCD contains front and back polarizer at right angles to each other. Without electrical excitation, the light coming through the front polarizer is rotated 90° in the fluid.
NEMATIC PHASE LCD §A nematic liquid crystal is a transparent or translucent liquid that causes the polarization (that is, the focusing in a plane) of light waves to change as the waves pass through the liquid. §The extent of the change in polarization depends on the intensity of an applied electric field. § Nematic comes from a Greek prefix nemato meaning threadlike and is used here because the molecules in the liquid align themselves into a threadlike shape. §Nematic liquid crystals are used in twisted nematic displays, the most common form of liquid crystal display.
TWISTED NEMATIC DISPLAY (TN DISPLAY) §A twisted nematic (TN) display is a common type of liquid crystal display ( LCD) that consists of a substance called a nematic liquid crystal that is confined between two plates of polarized glass. §The TN display takes advantage of the ability of the nematic substance to rotate the polarization of light beams passing through it. §Two polarizing filters, parallel planes of glass with their polarizing lines oriented at right angles with respect to each other, are positioned on either side of the liquid crystal. .
TWISTED NEMATIC DISPLAY (TN DISPLAY) • When light enters the display, it is polarized by the input filter. In the absence of an electric field, all the incoming light is transmitted. • This is because the light polarization is rotated 90 degrees by the nematic liquid crystal, and the light therefore passes easily through the output filter, which is oriented to match the 90 degree shift. • Most TN displays have a characteristic black on gray or black on silver appearance, and are suitable for use in alphanumeric readouts such as those found in wristwatches, cell phone displays, and some calculator displays. • The high resolution LCD panels typically found in handheld and notebook computers use a different technology.
CONSTRUCTION OF LCD §To make an LCD, we need to take two polarized glass pieces. The glass which does not have a polarized film on it must be rubbed with a special polymer which creates microscopic grooves in the surface. § It must also be noted that the grooves are on the same direction as the polarizing film. §Then, we need to do is to add a coating of nematic liquid crystals to one of the filters. The grooves will cause the first layer of molecules to align with the filter’s orientation.
CONSTRUCTION OF LCD §At right angle to the first piece, we must then add a second piece of glass along with the polarizing film. §Till the uppermost layer is at a 90 degree angle to the bottom, each successive layer of TN molecules will keep on twisting. §The first filter will naturally be polarized as the light strikes it at the beginning. § Thus the light passes through each layer and is guided on to the next with the help of molecules.
CONSTRUCTION OF LCD §When this happens, the molecules tend to change the plane of vibration of the light to match their own angle. §When the light reaches the far side of the liquid crystal substance, it vibrates at the same angle as the final layer of molecules. §The light is only allowed an entrance if the second polarized glass filter is same as the final layer.
WORKING OF LCD §The main principle behind liquid crystal molecules is that when an electric current is applied to them, they tend to twist. §This causes a change in the light angle passing through them. §This causes a change in the angle of the top polarizing filter with respect to it. So little light is allowed to pass through that particular area of LCD. §Thus that area becomes darker comparing to others.
WORKING OF LCD §For making an LCD screen, a reflective mirror has to be setup in the back. §An electrode plane made of indium tin oxide is kept on top and a glass with a polarizing film is also added on the bottom side. §The entire area of the LCD has to be covered by a common electrode and above it should be the liquid crystal substance. §Next comes another piece of glass with an electrode in the shape of the rectangle on the bottom and, on top, another polarizing film. It must be noted that both of them are kept at right angles. .
WORKING OF LCD §When there is no current, the light passes through the front of the LCD it will be reflected by the mirror and bounced back. §As the electrode is connected to a temporary battery the current from it will cause the liquid crystals between the common plane electrode and the electrode shaped like a rectangle to untwist. §Thus the light is blocked from passing through. Thus that particular rectangular area appears blank